Waves and your boat

At what size do waves get big enough to cause trouble for your boat.

Wave development

Waves are created by wind blowing across the surface of the ocean. Generally speaking, the stronger the wind, and the more time and distance the wind has to blow, the larger the waves generated. Wind in itself does not pose an enormous threat until it reaches quite sever strenght. Waves contain much more power and are of more concern for offshore sailors.

Wave characteristics

There are a lot of factors which influence sea state and how uncomfortable or dangerous the waves are:

• Wind speed (strength): the stronger the wind, the more power it will put into the waves and the bigger they will be
• Wind duration: the longer the wind blows, the bigger the waves
• Fetch: the longer the distance a wave travels, the bigger it will grow
• Wind direction: a stead wind direction will create a long swell from one direction. If the wind direction changes or meet a cross sea (waves from another direction), the sea will become “confused” (random waves appearing from unexpected angles) and might break. A breaking wave is one who’s base can no longer support it’s top and it collapses.
• Shape of the seabed: waves which have developed in deep water (water deeper than one-half of waves length) will be severely constrained when they reach shallow water (e.g. continental shelf in Bay of Biscay, shallows on the open ocean like Gorringe Ridge off west coast of Portugal). At the point where the depth of the water is one-half of the wave’s length, it begins to “feel” the bottom. The wave will slow down, grow taller and become shaped like peaks. These wave peaks reach a height where they become unstable and, moving faster than the water below, they break forward.

• Tide or current: the wind causes surface current of its own. A wind that has blown from a steady direction for several hours kicks up a current of about 2 percent of its speed. A 25-knot wind may create a 0.5-knot current. The wind blowing and the generated current against the direction of tide can create a much worse sea state than you might expect purely from the wind speed (1 knot of contrary current can double a wave’s height). In some parts of the world a wind force greater than 4 over tide can create unnavigable conditions (e.g. South-East coast of Africa). Moreover, if you consider the fact that 1 knot of current is worth 10kt of wind there are serious forces to go against.

Waves vs Swell

Waves caused directly by the local wind are called wind waves. Wind waves are short, choppy, and tend to break. Waves that travel outside of their generation area, and are no longer the result of the local wind, are called “swell”. Compared to wind waves, swell waves are longer with smoother crests.

Dangerous wave conditions

A cubic metre of water weighs one tonne, and water does not compress easily so when large waves start to land on deck or push the boat around, they can cause enormous damage. So, in most cases it is sea state rather than wind which need to be wary of.

Most sailors would be surprised at the minimal wave height needed to roll a boat. Andrew Claughton (who co-authored the University of Southampton, Department of Ship Science’s report) writes in Adlard Coles' Heavy Weather Sailing, “During the model tests that were carried out to investigate the problem, when the breaking wave was 30 percent of the hull length high, from trough to crest, it could capsize some yachts, while waves to a height of 60 percent of the hull length comfortably overwhelm all of the boats we tested.” So, the starting point for a breaking wave to be dangerous to rolling a boat is one that is only 30 percent high as the boat is long.

Your boat may be more or less susceptible to rolling than other boats based on its design, but the aspects to remember are that although your boat may be severly tilted over by the wave, for the wave to knock the boat down or completely roll over (even worse pitch pole) all of the below need to happen at the same time:

• Wave height must exceed 30% of the yacht length or must match the beam of the boat
• The boat needs to be broadside or oblique to the wave
• The boat needs to be struck by a breaking wave: unless the wave is breaking, the boat should ride up and over the wave regardless of the wave height in comparison to the boat length and the boat’s orientation to the wave

When the wave height reach 50% of the boat length, there is a high probability of capsize, with more than 60% the boat is going to be capsized every time! (of course only if broadsided and with breaking waves).

Weather forecast parameters to pay attention to

Wave height

Wave height is a distance measured from the trough to the crest of the wave. The wave forecasts typically show significant wave height (SWH), which is the average height of the highest one-third of all waves measured. A theoretical maximum wave height possible is approximately equal to two times of SWH!. So when the marine forecast says seas 3m you should expect waves up to 6m. Example for SWH of 3 meters:

Wave period

Wave period is a time that elapses between the passing of successive crests or troughs. The longer the wave period the bigger the wave and the stronger the wind that caused it. The shorter the period, the more chance of breaking waves to occur. Note that breakers can occur even with long wave period if the waves begin to “feel” the bottom or approach moderately steep shores.

As waves move farther away from their source region, their wave length and period gradually increase. Therefore, waves with long periods, greater than 10 or 12 seconds, are arriving from a distant source and are considered swell. Swell period determines how fast a swell will propagate across the open ocean.

Formulas:

• Wave speed in knots = 1.5 x period in secondes

Typical waves in the Atltantic have period of 6-8 seconds and travels at 12 knots (1.5 x 8). Period of 9-15 seconds might be an indicator of TRS existence. A swell with a period of 9 seconds will travel at 13.5 knots, while swell with a 15 second period travels at 22.5 knots!

Wave length

Wave length is a distance between successive crests or troughs. It is harder to measure than wave period and unfortunately it is not included in modern weather forecasts. You can use the below formulas for deep water to get rough estimation based on wave period.

Formulas:

• Wave length in meters = (g x square of period in secondes) / (2 x π) ~ 1.56 x square of period in secondes

Typical waves in the Atltantic have period of 6-8 seconds and wave length of 100m (1.56 x 8 x 8).

In shallow water the wave length and period decrease significantly as explained in this video.

Wind gusts

Not a wave parameter but quite important one as big difference between regular wind and gusts may cause shock stress and damage the rig and sails.

Predicting breaking waves

Predicting dangerous breaking waves is complex. The best information that most sailors will have to rely on is the weather report. The wave height and the wave length (or wave period) are the two pieces of critical information. Once you know them, you can determine whether the waves will have the potential to be breaking or not.

A wave will break, or collapse, depending on the wave’s steepness. The base of the wave (the wave’s length) can only support a wave of a certain height. If it exceeds that height, the wave becomes too steep and it collapses and breaks forward. This is called the wave’s breaking point.

• Rule 1: If a wave length is 7 times or less than the wave height, then the wave may break.
• Rule 2: When the water depth is approximately 1.3 times the wave height, the wave may break.

Determining boat danger zone

To determine dangerous wave heights and lengths for your boat, the two formulas need to be met:

• Wave Height >= 30% of Boat Length or match the beam of the boat
• Wave Length <= Wave Height x 7

You can use that formula as a general rule but note that the danger presented depends on the boat design and is unique to each vessel.

Example 1. Boat 40 feet long, 3.6m beam, significant wave height 1.8m/6feet (Hmax 3.6m), wave period 6 sec (wave length = 1.56 x 6 x 6 ~56m), deep water.

Using the minimum 30 percent wave height to boat length, if your boat is 40 feet (12m) long and 3.6m wide, then the wave-height danger zone starts at waves 3.6m high (0.3 x 12m), SWH of 1.8m/6feet. A wave 3.6m high could knock down the boat (throw boat over to 90 degrees or more) if caught somewhere near beam onto the wave as it breaks. Following the Rule of 7 or less, you know that a 3.6m wave could break only if the wave length is less than 25m (3.6m x 7), 4 sec wave period. So, these conditions should not present a danger to the boat in deep sea.

Example 2. Boat 40 feet long, 3.6m beam, significant wave height 2.5m/8feet (Hmax 5m), wave period 6 sec (wave length ~56m), deep water.

The boat is within wave height danger zone (5m Hmax > 0.3 x 12m = 3.6m) and not within wave length danger zone (56m wave length > 7 x 5m = 35m). So, these conditions should not present a danger to the boat.

Example 3. Boat 40 feet long, 3.6m beam, significant wave height 3.2m/10feet (Hmax 6.4m), wave period 6 sec (wave length ~56m), deep water.

The boat is within wave height danger zone (6.4m Hmax > 0.3 x 12m = 3.6m) but not within wave length danger zone (56m wave length > 7 x 6.4m = 45m). So, these conditions should not present a danger to the boat even though some may find these conditions worrisome. However, if the conditions are harder than predicted by 20%, it will become dangerous to sail in these conditions. It’s your call but a prudent sailor would consider this dangerous.

Example 4. Boat 40 feet long, 3.6m beam, significant wave height 4.2m/13feet (Hmax 8.4m), wave period 6 sec (wave length ~56m), deep water.

The boat is within wave height danger zone (8.4m Hmax > 0.3 x 12m = 3.6m) and within wave length danger zone (56m wave length < 7 x 8.4m = 59m). So, it might be dangerous for the boat to sail in these conditions.

Below table can be used as guidance to indicate potential danger to vessels with a closed cockpit configuration of ~100 feet or less. The table was developed for coastal waters (within 60 nm of shore) off the US Western Pacific coast by the National Weather Service. The specific values and conditions might be different to other waters such as Atlantic Ocean. However, an important message indicated by the table applies to all waters: for small vessels, danger increases when the wave period/length decreases (for a given wave height).

How to avoid breaking waves

In today’s world of satellite communication and more accurate weather forecasts, it is certainly easier to avoid heavy weather than before. Sailing up a sea storm is very hard, sometimes impossible. That is why it is important to plan, execute and monitor passages properly, with a good weather forecast in your hands and an alternative strategy in your mind.

You should generally stay in harbour if bad weather is predicted. But once you are out on the sea, far from a harbour, and the forecast predicts a deep low in your vicinity, you might not have enough time to avoid the system. You can attempt to escape as far away from the low as possible, and ensure that you are some distnace away from any shelving seabed which could increase likelihood of breaking waves. Other than that, you should plan passages to avoid unfavourable seasons, e.g. hurricane season that can create Tropical Revolving Storms (TRS) that must be avoided at all costs (North Atlantic and North Pacific: July - November; Bay of Bengal and Arabian Sea: June - November; South Pacific and South Indian Ocean: November - April).

How to cope with breaking waves

The most important tactic in copying with breaking sea is not to get caught beam on to the waves. Choose a storm tactic that will minimize this. For a typical moder cruiser, this would mean running downwind and for traditional vogayer with long keel probably heaving-to.

A really cool part of heaving-to is that the boat will leave a wake to windward. Breaking waves hit this “slick” and flatten out, thus reducing the wave action on the vessel. Modern lightweight boats generally don’t heave-to very well, and certainly not as well as a solidly built full-keel boat will. The sailplan and hull geometries of modern designs just do not let the boats lie stable to the wind. So you will have to decide depending on what works for your boat.

Fair Winds and Following Seas!

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